Yarn brakes of the so-called "platelet" type are known, as shown for example in Austrian Patent Specification No. 180,017 and German Patent Specification No. 968,222. In this type of yarn brake, two brake platelets are mounted with their opposing surfaces pressing against one another so as to apply a desired tension or braking force to a yarn passing between the brake platelets. Typically, either one or both of the brake platelets is mounted for movement within the brake housing, and the brake platelets are biased toward one another either magnetically or by means of springs.
In this type of brake mechanism, some provision must be made to insure that when the yarn is pneumatically threaded through the brake housing, the yarn will be properly positioned between the two opposing brake platelets. In some of the prior platelet-type brakes, a movable mechanical element has been provided for this purpose which is adapted to move one of the platelets away from the other to allow a yarn to be threaded therebetween. However, yarn brakes employing such movable mechanical elements are of relatively expensive design and are undesirable in that they require an additional working step to be carried out by the operator during a threading-up operation.
In the case of the yarn brake shown in the aforementioned German Patent Specification No. 968,222, two cooperating saucer-shaped brake platelets are mounted inside a relatively large opening or cavity in the brake housing. The edges of the two brake platelets are chamfered so that the two platelets form a V-shaped groove to allow the yarn to enter between the two platelets. However, because of the relatively large cavity provided in the brake housing, there is the possibility that when the yarn is threaded through the brake housing it will not reach the V-shaped groove so as to subsequently be brought in between the two brake platelets, but instead may bypass the two brake platelets and thus pass through the brake housing without any braking or tension force being applied to the yarn.
Similar problems with respect to the proper positioning of the yarn between the brake platelets following a threading operation are presented in the prior yarn brake arrangement shown in the aforementioned Austrian Patent Specification No. 180,017.
In another platelet-type yarn brake mechanism described in commonly assigned U.S. Pat. No. 3,945,184, one of the two cooperating braking members is formed as a part of the housing and comprises a generally flat inclined braking surface which surrounds the yarn passageway leading into the brake housing. A movable brake platelet is provided cooperating with the stationary braking surface for covering the yarn passageway and for thus applying tension to a yarn passing through the passageway and between the stationary braking surface and the movable brake platelet. A yarn bypass channel is formed in the brake housing leading from the yarn passageway and laterally alongside the stationary braking surface for receiving the yarn during the pneumatic threading of the yarn through the yarn passageway without the need for moving the movable brake platelet away from the stationary braking surface. When tension is subsequently applied to the yarn, as for example when the twisting spindle is restarted, the yarn is moved out of the bypass channel and into the proper position between the stationary braking surface and the movable brake platelet. Yarn brakes of this type have proven their worth in actual practice on many occasions. However, the specially shaped braking surface and yarn bypass channel which is required in this type of arrangement is relatively expensive to produce and undesirably increases the cost of the brake mechanism.